1. Field of the Invention
This invention relates to colour laser marking, more specifically to stabilizers for colour laser markable laminates and documents, especially security documents.
2. Description of the Related Art
Articles are laser marked in order to ensure product safety and authenticity. For example, packaging material of pharmaceuticals is laser marked to enable a consumer to know the genuineness of a product. Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various papers or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such articles and security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
Today, laser marking employed in the manufacture of security documents consists solely of a “black” laser marking method via the carbonization of a polymer, usually polycarbonate as disclosed in e.g. EP 2181858 A (AGFA). There has been considerable interest in being able to produce colour images through laser marking.
U.S. Pat. No. 4,720,449 (POLAROID) discloses a thermal imaging method for producing colour images on a support carrying at least one layer of a colourless compound, such as di- or triarylmethane, by conversion of electromagnetic radiation into heat. The laser beam may have different wavelengths in a range above 700 nm with at least about 60 nm apart so that each imaging layer having a different infrared absorber may be exposed separately to convert a colourless triarylmethane compound into a coloured form. However, there is no disclosure on stabilizers for preventing additional colour formation upon longer daylight storage, e.g. on top of a car's dashboard.
EP1852270 A1 (TECHNO POLYMER CO) discloses a laminate for laser marking comprising a layer with a multi-color developing laser marking thermoplastic polymer composition capable of producing markings having two or more different color tones by irradiating thereto two or more laser lights having different energies from each other. When the content of a thermosetting polymer in the thermoplastic polymer composition for laser marking lies within a specified range, the obtained laser-marking portions are free from discoloration.
WO9600262 A1 (NIPPON KAYAKU) discloses a laser markable composition comprising an energy ray curing resin, a leuco dye and a color developer. Discoloration of the laser marking composition was found to be suppressed or minimized when the color developer and the leuco dye exhibited low solubility in toluene, particularly a solubility below 5 w/v % at 25° C.
U.S. Pat. No. 4,602,263 (POLAROID) discloses a thermal imaging method for forming color images which relies upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift from colourless to colored, from colored to colourless or from one color to another. However, there is no disclosure on stabilizers for preventing additional colour formation upon longer daylight storage.
U.S. Pat. No. 5,955,224 (FUJI) disclose a thermally imagable composition comprising: (a) at least one near IR absorbing dye; (b) at least one hexaaryl-bimidazole compound; (c) at least one leuco dye; (d) at least one acid-generating compound; and (e) a polymeric binder which is preferably a polyvinyl butyral and brominated polystyrene; and optionally at least one UV stabilizer and/or at least one inhibitor of color formation.
There is still a need for colour laser markable laminates improved for preventing additional colour formation upon long daylight storage, especially at elevated temperatures, and having no or minor influence on colour formation by laser marking, e.g. high optical densities.